Can one determine cosmological parameters from multi-plane strong lens systems?

Strong gravitational lensing of sources with different redshifts has been used to determine cosmological distance ratios, which in turn depend on the expansion history. Hence, such systems are viewed as potential tools for constraining cosmological parameters. Here we show that in lens systems with two distinct source redshifts, of which the nearest one contributes to the light deflection towards the more distant one, there exists an invariance transformation which leaves all strong lensing observables unchanged (except the product of time delay and Hubble constant), generalizing the well-known mass-sheet transformation in single plane lens systems. The transformation preserves the relative distribution of mass and light, so that a `mass-follows-light' assumption does not fix the MST. All time delays (from sources on both planes) scale with the same factor -- time-delay ratios are therefore invariant under the MST. Changing cosmological parameters, and thus distance ratios, is essentially equivalent to such a mass-sheet transformation. As an example, we discuss the double source plane system SDSSJ0946+1006, which has been recently studied by Collett and Auger, and show that variations of cosmological parameters within reasonable ranges lead to only a small mass-sheet transformation in both lens planes. Hence, the ability to extract cosmological information from such systems depends heavily on the ability to break the mass-sheet degeneracy.Comment: 5 pages, matches the printed versio

The cosmological lens equation and the equivalent single-plane gravitational lens

The gravitational lens equation resulting from a single (non-linear) mass concentration (the main lens) plus inhomogeneities of the large-scale structure is shown to be strictly equivalent to the single-plane gravitational lens equation without the cosmological perturbations. The deflection potential (and, by applying the Poisson equation, also the mass distribution) of the equivalent single-plane lens is derived. If the main lens is described by elliptical isopotential curves plus a shear term, the equivalent single-plane lens will be of the same form. Due to the equivalence shown, the determination of the Hubble constant from time delay measurements is affected by the same mass-sheet invariance transformation as for the single-plane lens. If the lens strength is fixed (e.g., by measuring the velocity dispersion of stars in the main lens), the determination of $H_0$ is affected by inhomogeneous matter between us and the lens. The orientation of the mass distribution relative to the image positions is the same for the cosmological lens situation and the single-plane case. In particular this implies that cosmic shear cannot account for a misalignment of the observed galaxy orientation relative to the best-fitting lens model.Comment: TeX, 11 pages, submitted to MNRA

U(g)-finite locally analytic representations

In this paper we continue the study of locally analytic representations of a $p$-adic Lie group $G$ in vector spaces over a spherically complete non-archimedean field $K$, building on the algebraic approach to such representations introduced in our paper "Locally analytic distributions and p-adic representation theory, with applications to GL_2." In that paper we associated to a representation $V$ a module $M$ over the ring $D(G,K)$ of locally analytic distributions on $G$ and described an admissibility condition on $V$ in terms of algebraic properties of $M$. In this paper we determine the relationship between our admissibility condition on locally analytic modules and the traditional admissibility of Langlands theory. We then analyze the class of locally analytic representations with the property that their associated modules are annihilated by an ideal of finite codimension in the universal enveloping algebra of G, showing under some hypotheses on G that they are sums of representations of the form $X\otimes Y$, with X finite dimensional and Y smooth. The irreducible representations of this type are obtained when X and Y are irreducible. We conclude by analyzing the reducible members of the locally analytic principal series of SL_2(\Qp)

Mass-sheet degeneracy, power-law models and external convergence: Impact on the determination of the Hubble constant from gravitational lensing

The light travel time differences in strong gravitational lensing systems allows an independent determination of the Hubble constant. This method has been successfully applied to several lens systems. The formally most precise measurements are, however, in tension with the recent determination of $H_0$ from the Planck satellite for a spatially flat six-parameters $\Lambda CDM$ cosmology. We reconsider the uncertainties of the method, concerning the mass profile of the lens galaxies, and show that the formal precision relies on the assumption that the mass profile is a perfect power law. Simple analytical arguments and numerical experiments reveal that mass-sheet like transformations yield significant freedom in choosing the mass profile, even when exquisite Einstein rings are observed. Furthermore, the characterization of the environment of the lens does not break that degeneracy which is not physically linked to extrinsic convergence. We present an illustrative example where the multiple imaging properties of a composite (baryons + dark matter) lens can be extremely well reproduced by a power-law model having the same velocity dispersion, but with predictions for the Hubble constant that deviate by $\sim 20$. Hence we conclude that the impact of degeneracies between parametrized models have been underestimated in current $H_0$ measurements from lensing, and need to be carefully reconsidered.Comment: Accepted for publication in Astronomy and Astrophysics. Discussion expanded (MSD and velocity dispersion, MSD and free form lens models, MSD and multiple source redshifts